Slice the sourdough loaf and let sit on the cutting board
Mix the eggs, milk, vanilla, salt, and cinnamon-sugar mix in a glass pie pan or square baking dish.
Heat a large frying pan over medium heat and melt butter in the pan to cover the bottom (about a Tablespoon)
Dip a piece of bread into the bread mixture, making sure both sides of the sourdough bread are covered.
Place each piece of bread into the pan and cook until golden brown, then flip and cook until golden brown on the second side
Move cooked french toast slices to a serving dish and repeat process with remaining slices of bread.
Notes
If you aren't going to serve the french toast right away, you can keep it warm in oven set on it's lowest temp. I wouldn't hold it more than an hour as you don't want to dry out the french toast.Be careful not to crowd the pan. It will take longer for the pieces to cook and they will likely cook and brown unevenly. I suggest no more than 2-3 slices of bread at a time.You can easily substitute plant based butter and almond or soy milk to make this dish dairy free.